IEEE 802.11 standards have been developed as the standards for Wireless Local Area Network (WLAN) and are widely supported and applied; meanwhile the demands for a large scale enterprise network, a large community network or even a Metropolitan Area Network (MAN) are brought forward. In a large WLAN, a great number of Access Points (APs) have to be deployed, each of which covers a specified area, to provide a large continuous coverage. Hence it is required that a Mobile Terminal (MT) shall be able to perform a handoff from one AP to another AP while moving around in the continuous coverage and still maintains service continuity. The basic handoff flow of an MT includes the following steps.
1) The MT scans wireless channels. The MT measures indexes of the wireless channels, e.g., the signal intensity, while scanning the wireless channels. Then the MT performs a handoff from one AP to another AP based on the measured indexes. For example, the MT may choose the channel with the best wireless channel quality as the destination AP in the handoff process.
2) The MT initiates a reassociation request. Having received the reassociation request initiated by the MT, the destination AP has to enable the system to delete the association between the MT and the origination AP, which was established before the handoff. Then the destination AP establishes a new association with the MT.
3) The data services of the MT are forwarded by the destination AP after the new association has been established. Meanwhile, the MT continues to scan and measure indexes of other wireless channels to determine whether it is necessary to perform a handoff to a new destination AP, and to determine the new destination AP when a handoff is needed.
Conventionally, the WLAN supports streaming media, voice services as well as common data services. When the MT performs a handoff, it is necessary to minimize the influence on the services caused by the handoff, and basic requirements on handoff include that:
a) the handoff period should be short enough so that the services will not be interrupted;
b) the quality of service (QoS) after the handoff should be better than the QoS before the handoff.
When a WLAN includes multiple APs in the network and an MT performs a handoff from one AP to another AP, there are two ways to perform the access control on the network infrastructure which include a distributive access control and a centralized access control.
As shown in FIG. 1, in a distributive access control which is performed by APs, the handoff of the MT from one AP to another AP is performed via exchanging information between the two APs. The IEEE 802.11f which is recommended by the IEEE P802 Working Group defines a protocol framework for the communication between APs, i.e., Inter-AP Protocol (IAPP), based on which the inter-AP handoff of the MT can be performed. The protocol defines the inter-AP interaction process involved in the association or reassociation of the MT with an AP. Furthermore, a Radius server for IAPP service is added into the infrastructure to ensure a safe communication between APs, which requires the APs to implement Radius protocol after being powered on and to register at the Radius server. An AP can look up the IP addresses of a corresponding AP on the Radius server by using the Basic Service Set Identification (BSSID) of the AP and starts IP layer based communication with the AP with security information from the Radius server. IEEE 802.11f adds extensions to the Radius protocol in order to support the functions.
As shown in FIG. 2, some manufacturers have brought forth a lightweight AP system architecture, i.e., some of the functions of Media Access Control (MAC) on the AP are centralized in a exchange, usually referred to as Wi-Fi exchange or a router, usually referred to as Access Router on an upper layer. The IETF CAPWAP Working Group divides the centralized WLAN into three types of architectures: Remote MAC, SPLIT MAC and Local MAC, in which the Wi-Fi exchange or the Access Router on the upper layer are generally referred to as an Access Controllers (ACs). In the Remote MAC architecture all MAC functions of an AP are transferred to the AC; in the SPLIT MAC architecture the MAC functions which are not sensitive to time are transferred to the AC; and in the Local MAC architecture the MAC functions are still performed by an AP, but managed by an AC. The lightweight AP equals SPLIT MAC architecture and related manufacturers have released a Lightweight AP Protocol (LWAPP) to IETE.
The most prominent advantage of the centralized architecture is that the APs can be managed and controlled in a centralized manner by an AC, especially in the inter-AP handoff process of the MT, in which the APs need not to communicate with each other directly, but to interact with the AC only. In an LWAPP based architecture, the control frames on the MAC layer of the air interface, e.g., the association request and reassociation request, are handled by the AC directly, therefore the access control in the handoff process is performed inside the AC entity.
In either the distribution system or the centralized system, the handoff decision in the WLAN is made by the MT according to the signal intensity collected by the MT, and the MT needs to scan all channels, which requires a long time, hence the conventional handoff scheme in which the MT scans all channels and makes handoff decision without considering other indexes of the channels may result in a low QoS e.g., congestion at an AP which causes a low QoS or even malfunctions including service unavailable or service interruption.